I have wrenched my greymatter to think of a cause, but I am at a loss ...
Despite functioning properly on some boards, the same Swimlane setup does not show on a select few boards.
Swimlane Setup:

Board 1 (Works fine):

Board 2 (Does not Work):

Certainties:
- Our boards are each filtered to only show one specific Issue Type.
- Both boards have the correct Swimlane Query syntax.
- All project Issue Type share common Fields such as Fix Version and Parent.
- The Issues Types on both boards have Fix Version as either empty or set.
- The white highlight color is queried: <fixVersion is empty>
- Board 1 Issue Type does not "share" a Workflow with another board.
- Board 2 Issue Type does "share" a Workflow with a "twin board".
EDIT ADD:
- Everything is in the same project.
- We use a team-related task sorting system where each team has their respective Milestone (Release Version/Fix Version) Epic that all team-related tasks have their Parent. An Automation then sets the same Fix Version to the task based on the Parent Epic's Fix Version.
From what I've been able to assess is that this problem may have something to do with the task's Workflow. As we use staggered completion stages for certain Issue Types, they will have a set of Statuses used on one board, whereas the latter stage Statuses are on its own separate board.
Board 2 Column+Status Setup:

- Note: Board 2 filter does not show the Ready for Environment Design status.
Board 2 "TWIN" Column+Status Setup:

Board 1 Issue Type Workflow:

Board 2 Issue Type Workflow:

TL;DR
Swimlanes not working properly, and I suspect it is due to boards "sharing" a Workflow, though I can't for the life of me understand why.
Any insight as to why this happens is greatly appreciated. I am open to try fix suggestions.